The invention relates to a method for supplying liquids by means of a pump combination consisting of two single oscillating displacement pumps wherein during the displacement operation of the one pump the other pump is started and pre-compresses the supplied medium with the outlet valve closed, then is stopped while holding the pre-compression final pressure reached, and is continued with its stroke while supplying the supplied medium only when the corresponding other pump ends its supply. Therein the pulsing, when transferring the supply from the one pump to the other, may be kept very small.
Such and similar pumps are known from the documents AU 1 450 400, U.S. Pat. No. 5,141,408, U.S. Pat. No. 4,127,360 and U.S. Pat. No. 4,191,309.
The known single pump comprises a suction stroke which is faster than the compression stroke, such that with movements offset by 180.degree. of the two pumps, a time reserve is present between the end of the suction stroke of the one pump and the beginning of the subsequent supply stroke of the same pump, with the time reserve bridged by a stop of the displacing member after the pre-compression stroke. During the pre-compression stroke, the displacing member performs its pressure stroke with the outlet valve closed, until a required pressure is reached for the pressure medium enclosed in the supplying cylinder, whereupon the movement of the displacing member is stopped while maintaining this pre-compression final pressure.
In the pump described in U.S. Pat. No. 1,450,400, referring to FIGS. 1 and 2, the movement of the displacing members is generated by means of electrical step motors acting upon spindles. During the pre-compression stroke of the one displacing member, the associated step motor is supplied with electrical power which is associated with a slipping torque corresponding with the required final pressure. After having reached the required pressure, the step motor starts to slip while maintaining the pre-compression pressure reached, wherein the displacing member comes to a stand still. When the other single pump supplying to the user reaches the end of its supply stroke, the outlet valve of the single pump standing still and presented with the pre-compression stroke is opened, and simultaneously the electrical power for the step motor is increased, such that this pump continue its pressure stroke while supplying the medium to the user.
The drive unit and the control of this pump system is costly and complicated and there is no automatic adaption or modification of the final pressure of the pre-compression when the supply pressure changes.
In the pump system described in U.S. Pat. No. 5,141,408 the pre-compression pressure is sensed by a pressure sensor. When a preset pressure is reached, the sensor supplies a signal by means of which the drive unit, in this case a hydraulic drive, is deactivated for the hydraulic cylinder driving the displacing member stops. Also, in this case, the final pressure of the pre-compression has to be adjusted to the required value from the outside, and has to be adapted to the supply pressure acting in the ducts to the user.